In recent years, we have witnessed an enormous increase in applications of quantitative isotopic measurements that have contributed to solving problems related to chemical pollution. Some fundamental concepts have been laid out in an excellent introduction on this developing technique. They all originate from the theory of isotopic chemistry of chemical and biochemical reactions, however, because of the specificity of environmental field work, some simplifications have been necessary. For example, although it is usually quite simple to study a chemical or even a biochemical reaction at the level of milligram quantities of reactants, in environmental studiesmuch smaller quantities are usually available for analysis. Although technological advances in isotopic mass spectrometry now allow for measurements of such small amounts, they are usually at the expense of having to average isotopic composition of all atoms of a given element present in the reactant molecule at different positions (i.e. compound specific isotopic analysis, CSIA). The abovementioned differences, as well as the tradition of representing data and the scientific jargon used, both exemplified in Table 1 below, have impeded communication between isotopic chemists and environmentalists. It thus seems important now to recall some of the assumptions used in chemical foundations that might have bearing on the interpretation of environmental results. In the author’s opinion, below are a few important issues that, although raised in the original contribution, are sometimes forgotten in the follow up reports.